1. Field of the Invention
The present invention relates to a peeling method of a layer to be peeled, especially, a peeling method of a layer to be peeled containing various elements. In addition to that, the present invention relates to a semiconductor device comprising a circuit composed of a thin film transistor (hereinafter referred to as TFT) transferred by sticking a separated layer to be peeled to a substrate and its manufacturing method. The invention relates, for example, an electrooptical device such as a liquid crystal module, a light emitting device such as an EL module, and an electronic apparatus containing such devices as components therein.
The term semiconductor device, in this specification, covers any device as a whole capable of functioning by utilizing semiconductor characteristics and an electro optical device, a light emitting device, a semiconductor circuit, and electronic apparatuses are all included in the semiconductor device.
2. Description of the Related Art
Recently, attention has been paid to technology of composing a thin film transistor (TFT) using a semiconductor thin film (the thickness of about several to several hundred nm) formed on a substrate having an insulating surface. The thin film transistor is widely applied to electronic devices such as IC and an electrooptical device. Especially, its development as a switching element for an image display apparatus has been urged.
Among a variety of expected applications of such an image display apparatus, above all, utilization for mobile devices draws attention. Today, although a glass substrate, a quartz substrate and the like are used in many cases, they have disadvantages that they are easy to crack and heavy. Further, in terms of mass production, the glass substrate, the quartz substrate and the like are difficult and unsuitable for enlargement. Therefore, a substrate having flexibility, such as a flexible plastic film has been tried to form a TFT element thereon.
However, since the heat resistance of the plastic film is low, the highest temperature for process is required to be low and consequently, it is presently impossible to form any TFT with electric characteristics as excellent as those formed on a glass substrate. Hence, no liquid crystal display apparatus and light emitting element using a plastic film has been actualized.
Further, a peeling method for peeling from the said substrate a layer to be peeled existing on a substrate through an isolation layer, has been already proposed. For example, a technique described in Japanese Unexamined Patent Publication No.10-125929 and Japanese Unexamined Patent Publication No. 10-125931 is a method for separating a substrate by forming an isolation layer of amorphous silicon (or polycrystalline silicon) and releasing hydrogen contained in the amorphous silicon by radiating laser beam through a substrate to form voids for substrate separation. In addition to that, in Japanese Unexamined Patent Publication No.10-125930, there is a description that using this technique, a liquid crystal display apparatus is completed by sticking a layer to be peeled (in the specification, called as a transferred layer) to a plastic layer.
However, by the above-described method, it is essential to use a substrate with high translucency. In order to transmit energy through the substrate and supply energy sufficient to release hydrogen contained in the amorphous silicon, relatively intense laser beam has to be radiated, resulting in a problem that the layer to be peeled is damaged. Further, by the above-described method, in the case an element is formed on an isolation layer, if heat treatment at a high temperature is carried out in the element fabrication process, hydrogen contained in the isolation layer is diffused and decreased. As a result, there is a possibility of insufficient peeling, even if laser beam is radiated to the isolation layer. Subsequently, attributed to retention of the quantity of hydrogen contained in the isolation layer, there occurs a problem that the process after isolation layer formation may be limited. Also, in the above-described specification, there is a description that in order to prevent damages in the layer to be peeled, a light shielding layer or reflecting layer is formed. In such a case, it becomes difficult to fabricate a transmission type liquid crystal display device. Additionally, by the above-described method, peeling of the layer to be peeled having a large surface area becomes difficult.